function uniteUnique( arr ) {
  const united = [];
  for ( let i = 0; i < arguments.length; i++ ) { //iterate through arguments arrays
    for ( let a = 0; a < arguments[ i ].length; a++ ) { //iterate through each argument array
      united.push( arguments[ i ][ a ] );
    }
  }
  return arr = [ ...new Set( united ) ]; //new Set returns set of unique values, spread operator transforms it into array
}

console.log( uniteUnique( [ 1, 3, 2 ], [ 5, 2, 1, 4 ], [ 2, 1 ] ) );
